The effects of chronic exercise and stress on the myocardium and its nutrient vessels are poorly understood. Recently our laboratory has published several studies on mechanical and biochemical adaptations in hearts of physically trained animals. The role of the coronary vasculature and coronary collaterals in these mechanical adaptations have not been elucidated. We propose to intensively study the effects of exercise on both the anatomy as well as functional capacity of myocardial vessels in the rat and dog. We will determine whether chronic exercise causes increases in the mass of coronary vessels, and whether cessation of this exercise is accompanied by regression. Furthermore, we propose to study whether exercise induces development of coronary collaterals and whether these collateral channels will then have the capacity to protect the myocardium when the main coronary vessel is occluded. Before exercise programs can be intelligently advocated for either rehabilitation or prophylaxis in coronary artery disease, these results in experimental animals should be known.